1. Field of the Invention
The present invention relates to novel apparatus and processes for the injection of oxygen into a rotary kiln. More particularly, the present invention relates to apparatus and a processes which significantly improve combustion in a rotary kiln used for the calcination of minerals such as cement, lime, dolomite, magnesia, titanium dioxide, and other calcined materials
2. Brief Description of the Related Art
The introduction of oxygen into a combustion space, e.g., a furnace, is used in a variety of industries for the enhancement of the combustion process. To date, the use of oxygen in rotary kilns has been applied in three main ways, well documented in literature: introducing oxygen into the primary air, i.e., into the main burner; the utilization of an oxy-burner in addition to a standard air burner; and oxygen lancing into the rotary kiln, particularly in a region between the load and the flame, for improved flame characteristics. One of the more documented uses of oxygen in rotary kilns is described in Wrampe, P. and Rolseth, H. C., "The effect of oxygen upon the rotary kiln's production and fuel efficiency: theory and practice", IEEE Trans. Ind. App., 568-573 (November 1976), which indicates that production increases above 50% produce excessive temperatures into the kiln, but, below this level, kiln operation takes place without major problems.
Each method of introducing oxygen into the calcining plant has its advantages, as well as certain disadvantages. Thus, the total amount of oxygen which can be introduced into the primary air is limited, since the primary air-type kilns constitute only a relatively small proportion (5-10%) of modern rotary kilns. Therefore, in order to significantly increase the amount of oxygen introduced into the kiln, a large concentration of oxygen into the air-fuel mixture is necessary. This leads to potential safety problems, since the fuel is in contact with significantly enriched air prior to its arrival into the combustion space, and therefore it can burn too early, or even cause explosions. The use of oxy-burners, while offering the potential of improved overall heat exchange to the load, can require using a large amount of high-quality, high-cost fuel within the oxy-burner for a significant impact on product, e.g., clinker, formation. At the same time, the impact of the oxy-flame on the main fuel combustion may be limited.
The introduction of oxygen into the primary air in a kiln drastically limits the amount of oxygen which can be introduced into the kiln, and also only uniformly improves combustion in the entire kiln volume. The advantages of using oxygen are therefore diminished by the overheating of the kiln walls which results from the uniform increase in heat transfer to the kiln volume, without preferentially transferring heat to the load. The same effect is obtained when oxygen lances are installed into the main burner.
The use of a separate oxy-burner represents a more involved method to increase the thermal transfer to the load, which typically requires increased quantities of quality fuel, such as natural gas. The use of lances, although potentially leading to improvements in the flame patterns, has only limited capabilities. Thus, when utilizing lances located in the main burner, the flame radiates in all directions with the same intensity, providing a large portion of the heat directly to the walls, thus overheating the kiln walls. The high grade heat provided by the oxy-flame is therefore poorly used, with accompanying losses in the kiln's efficiency. Placement of the lances between the burner and the flame has partially corrected this problem, but results in mixing the fuel and the oxygen further in the kiln, which leads to a longer, less radiant flame. Furthermore, the flame tends to touch the kiln walls in a region where it overheats the wall, without great thermal impact on the load.
The prior use of lances between the flame and the load therefore represents a relatively common method of enriching the combustion air. While this oxygen injection method can have a beneficial effect on the combustion process in the kiln, it has not had the capability of locally optimizing the heat transfer to the load, mainly because the fuel is fired in the same manner as in the absence of oxygen. This method also has a limited effect in situations where dust insulation is important, or when the fuel quality is very poor. Lances have been investigated by previous patents, including U.S. Pat. No. 5,572,938, U.S. Pat. No. 5,007,823, U.S. Pat. No. 5,580,237, and U.S. Pat. No. 4,741,694. Oxygen burner use in a dolomite kiln has been proposed by U.S. Pat. No. 3,397,256.
Finally, U.S. Pat. No. 4,354,829 describes mixing air and oxygen in a separate pipe, and introducing it through the moving walls of a rotary kiln. This approach has a number of problems, among which are the difficulty of creating a leak free plenum which rotates with the kiln, and the difficulty of installing tubes into the kiln. Indeed, introducing the air-oxygen mixture in the manner suggested by U.S. Pat. No. 4,354,829 results in unfavorable combustion characteristics, because the location at which the mixture is introduced may actually impede the combustion process. Additionally, the air introduced in the rotary kiln is cold, therefore introducing additional stresses in the rotary kiln which can damage its very expensive structure, etc.
The general use of oxygen in rotary kilns has already been shown to increase production, starting with the work of Gaydas, R. A., "Oxygen enrichment of combustion air in rotary kilns," Journal of the PCA R & D Laboratories, 49-66 (September 1965). This report presents test results from a period between 1960 and 1962. Gaydas mentions that Geissler suggested that oxygen be used for clinker production as early as 1903.